We have been studying a post-translational modification of tubulin (the protein building block of microtubules) which we think may regulate microtubule assembly or function. This involves the reversible enzymatic addition of a tyrosine residue to the C-terminus of the alpha chain. An improved procedure has been devised for purifying the enzyme which adds tyrosine. A separable detyrosylating enzyme has been purified 500-fold from brain and has been shown to possess some distinctive properties unlike those of other carboxypeptidases. Attempts have been made to reconcile our previous, apparently contradictory, findings that membrane bound tubulin from brain is isolated without any C-terminal tyrosine, yet tubulin from a particulate fraction from cultured neuronal cells becomes tyrosylated in vivo.